I wouldn't call it a decline at all, I only highlighted this area because it was particular strong in terms of significance and cultural output. You could argue that a golden age was followed by a silver age until 1933, as you mentioned.

Quite a few people argued since then that the rise of fascism was surprising because of the intellectual environment, but that it was a actually a pre-requisite for it to happen that way. If that is accurate or not, is a different matter but one could make a point that the "German question" combined with the propensity for total, systemic philosophical systems in German culture contributed to the uniqueness of what happened during 1933-45.